The present invention relates generally to stuffing box packing assemblies for valve stems, shafts and piston rods, and more particularly to improved designs for conventional sized five ring stuffing box packing assemblies including a high density compact five ring stuffing box packing assembly.
In the past, the five-ring stuffing box packing assembly shown in cross section in FIG. 1 has been used in stuffing boxes for valves where high performance results are not required. This five-ring stuffing box packing assembly is a flat combination set consisting of three graphite die-formed tape rings 2, 4 and 6 placed together in the center of the set with braided end-rings 8 and 10 being positioned at the top and bottom of the five-ring set. These five rings are mounted in a stuffing box 12 for sealing a valve stem 14. Graphite die-formed tape rings are low density rings formed from flexible graphite tape which are deformed against the stuffing box or valve stem when a gland (not shown) is tightened. These graphite die-formed tape rings are typically produced at a density of about 1.1 g/cc. A graphite tape having a density of about 0.7 g/cc density is the lowest flexible graphite tape density known to the inventor of the present application that a manufacturer can purchase to produce flexible graphite tape rings. A 0.7 g/cc finished ring is simply a spiral wrapping of the 0.7 g/cc flexible graphite tape with the tape ends adhered in place. In many cases, these rings must be split to facilitate installation into a valve stuffing box. If the ring is a spiral wrapping only of flexible graphite tape, cutting through one side of a ring to enable it to be slipped around a valve stem causes the individual tape wrappings to delaminate and separate. However, if the spiral wrapping of tape is compressed in a die-forming operation under a comparatively low amount of pressure, the tape wrappings form an accordion fold-type bond between the individual layers that enables the rings to be easily split and handled for installation purposes. Compression of the tape in the die forming operation increases the density of the die formed ring to a level above about 0.7 g/cc, normally about 1.1 g/cc.
Flexible graphite has a negative functional attribute in that it will extrude through fine clearances as a result of heavy compressive forces. To control and prevent extrusion, the anti-extrusion braided end rings 8 and 10 were conventionally positioned outside the flexible graphite rings 2, 4 and 6. These braided end rings are often formed from a soft carbon fiber braided packing material.
The five-ring "flat" combination packing assembly arrangements are very simple and easy to assemble. All components are square in cross-sectional shape, and it is a well known principle in industry that the three graphite die-formed tape rings are placed together in the center of the packing assembly with the braided end-rings being positioned at the top and bottom ends. As a result of the widespread use of the five-ring flat combination packing assembly, many valve designs in the industry are adapted for use with a stuffing box depth sufficient to accommodate only the five packing ring cross sections of the five ring flat combination. This stuffing box depth is presently the predominant market trend.
Although the flat, five ring stuffing box packing assembly is simple and easy to manufacture and install, it has limitations in expansion efficiency, resistance to abrasive damage caused by stem scale build up, valve stem friction and sealability properties.
The stuffing box packing assembly of U.S. Pat. No. 4,328,974 to Richard E. White et al., the disclosure of which is herein incorporated by reference, was developed to provide enhanced radial expansion and sealability characteristics for valves, such as petrochemical and power industry valves where high performance results are required. This eleven ring packing assembly, shown in cross section in FIG. 2, incorporates uniquely designed low density graphite Preform rings 14 and 16 mated to higher density graphite Adapter rings 18 and 20 in such a way that one pair of these rings has its radial expansion properties oriented toward the outside sealing surface, and the other pair of rings is likewise oriented toward the inside sealing surface. The Adapter and Preform rings are composed of flexible graphite tape layered in either a spiral-wrapped or a laminated format before die-forming. The adjacent mating face surface angles 22 and 24 of the Preform and Adapter rings are typically 45 degrees and 60 degrees respectively. The combination effect of mating differing density material at differing angles is to enhance the radial expansion characteristics of the packing assembly. This enhanced radial expansion ability enables a single packing assembly to cover a wider range of cross-sectional stuffing box clearances than would be possible with more conventional designs, and the enhanced expansion ability also leads to a higher degree of sealing efficiency.
The Preform 14, 16 and Adapter 18, 20 rings are composed of graphite, usually and typically of the flexible graphite tape material type. The Preforms 14, 16 have a density range of about 0.5 to about 1.4 g/cc. The Adapter rings 18, 20 have a density range of about 1.4 to about 1.7 g/cc. The remaining seven rings are present to provide important supporting functions. The four metal or GYLON.RTM. spacer rings 25, 26, 28 and 30 ensure that loading forces are distributed evenly around the compression surfaces of the paired Preform 14, 16 and Adapter rings 18, 20. The three braided rings 32, 34 and 36, positioned at the top, the center, and in the bottom of the packing assembly, are present to perform collectively as resiliency rings, anti-extrusion rings, and wiper rings. The braided rings 32, 34, 36 are compressible carbon or graphite braided material.
U.S. Pat. No. 4,328,974 also discloses an alternate seven ring packing assembly shown in cross section in FIG. 3. The principal difference of the seven ring packing assembly from the eleven ring packing assembly of FIG. 2, is that the Preform rings 14 and 16 have been combined into a single, reduced height Preform ring 38 which contains both the OD and ID angled face features 40 and 42 respectively of the Preform rings 14 and 16. The consolidation of the two Preform rings (14, 16) of FIG. 2 into the single ring 38 eliminates the need for two of the spacer rings (25, 26, 28, 30) and one of the braided rings (32, 34, 36) of FIG. 2. Thus, in addition to the Preform ring 38, the seven ring packing assembly has two flexible graphite Adapter rings 44 and 46, two spacer rings 48 and 50, and two compressible braided carbon or graphite rings 52 and 54. The seven ring packing assembly of FIG. 3 can be used to seal valves that have shallower stuffing boxes than would be possible with eleven ring assemblies. This feature of being able to seal the shallower stuffing boxes, however, was obtained with a compromise in sealing effectiveness. The reduced amount of Preform ring material used in this approach also reduced the seven ring packing assembly's comparative sealing effectiveness.
The eleven and seven ring assemblies of U.S. Pat. No. 4,328,974, in side-by-side comparative functional testing with the flat five-ring packing assembly of FIG. 1, have proven to be the best functional product for use in critical process valves where leakage could lead to significant process unit down-time or environmentally hazardous conditions. However, the complexity, cost, packing assembly height and installation requirements of the eleven and seven ring assemblies have proven disadvantageous.